1. Field of the Invention
The present invention relates generally to electrical motors and more particularly to an improvement in the commutation loop for a brushless D.C. motor wherein enhancement of the torque characteristics of the motor is provided.
2. Description of the Prior Art
Brushless Direct Current (BDC) motors are well known in the art.
To make the motor rotate through a complete revolution, it is necessary to commutate the windings as a function of rotor position. Commutate means to change the direction of current in the proper coils at the proper time; brush type motors do this by the arrangement of the brushes and the commutation bars. For BDC motors, electronic switches are added along with a Rotor Position Sensor (RPS).
FIGS. 1 through 6 illustrate waveforms, circuits and the sequencing used in electronic commutation of BDC motors. This information is well known in the prior art and is included to assist in a better understanding of the present invention.
FIG. 1 schematically shows a typical brushless motor wound with three phases and the voltages seen between the phases when the motor is run as a generator of constant speed. Note that the motor is wound to provide overlapping, sinusoidal 3 phase voltages, electrically spaced 120.degree. apart. In this example, the electrical degrees equal mechanical degrees; that is, the electrical spacing of the phase voltages corresponds to the rotor's physical position. Because the rotor has two poles, increasing the number of poles will increase the number of electrical cycles for each complete rotation of the rotor. The points of North/South balance for each winding occur where the voltages go through zero and reverse polarity. It is the method and type of winding, as well as the geometric and physical characteristics of the rotor and stator that create sinusoidal shape of the terminal voltage, the Back EMF (or BEMF) of the motor. The torque produced by a motor with a given winding and physical geometry is directly related to the voltage it produced when the rotor is externally driven, or when the motor is used as a generator. In fact, the motor torque constant, K.sub..dagger., and the motor voltage or Back EMF constant, K.sub.b, are equal when K.sub..dagger. is expressed in Newton.cndot. Meters per Ampere and K.sub.b is expressed in Volts per Radian per Second: EQU K.sub..dagger. =K.sub.b
This applies not only to the motor constants given in a manufacturer's data sheet, but also to the waveshape throughout the commutation cycle. In others words, if the BEMF waveforms are viewed as a function of rotor position on an oscilloscope, then, when a constant current is applied to the motor, the torque as a function of rotor position will vary in a similar manner, ignoring armature reaction effects. Such is illustrated in the waveforms of FIGS. 2 and 3.
There is a logical way to decide when to commutate a brushless motor in general cases. It is known that commutating at the zero crossing of the BEMF waveform is not a good place to start since there is no resultant torque no matter how much current is injected into the phase. Peak torque per unit current for a running motor is achieved at the peak of the BEMF waveform and it is desirable that the motor run smoothly, i.e., to transition smoothly between commutation cycles.
The commutations points for the motor in FIG. 3 are shown in FIG. 2 at the beginning of the shade areas. These commutation points center the peak of the BEMF waveform in the commutation zone, and provide equal sharing of the motor phases in the process of producing torque.
Due to the commutation points, however, the variation in the K.sub..dagger., of the motor is approximately 50% for the BEMF waveshape shown. That means that for a constant current input, the torque output over each commutation zone will vary by 50%. In some applications, such as ventilators or pumps, this may be acceptable. To improve this variation of torque during commutation, called torque ripple, the scheme as shown in FIG. 3 may be used.
In this case, commutation occurs twice as often during one revolution by using the negative half of the BEMF waveform as well as the positive half. The torque now falls approximately 13% below the peak. For a three phase motor, with the simple commutation scheme shown, this is the best that can be accomplished with the BEMF waveform of this particular motor.
The torque output of the motor can be seen effectively as a ripple which follows the application of the voltage to the stator coils of the brushless D.C. motor. For a given load when the motor is driven toward its limits (that is, maximum currents applied to the stator coils) the rotor will have a tendency to stall as the load increases. This stall usually occurs at a minimum torque point in the torque curve. In the prior art, this problem has been overcome by utilizing a larger motor to provide more torque to drive a given load or alternatively to enhance the characteristics of the motor by utilizing more exotic magnetic materials therein. In either case, additional costs are incurred to drive a given load to thereby meet the load driving specification required.
In brushless D.C. motors it is necessary to commutate the applied voltage so that only peaks of the applied A.C. multiphase voltage waves are applied to the coils of the stator of the motor in order to properly drive the rotor. In order to accomplish such commutation, means is provided to sense the rotor position (usually Hall effect devices) and to activate appropriate switches (usually transistors) to apply the voltage to the proper coils of the stator at the proper times. The sensor devices are positioned at pre-determined positions angularly about the rotor and as the rotor passes a sensor position it may be viewed as going from a non-active to an active position thereby providing an output which may be used to generate a signal for activating a switch. Such may be viewed as a change of state of the sensor device.
One such form of commutation which may be used is illustrated in FIGS. 4 and 5 and Table 1 of FIG. 6 and is referred to as a six sequence commutation. Six sequence commutation takes advantage of the three phases as shown in FIG. 3. Looking left to right, a positive or negative peak occurs in one of the phases every 60 electrical degrees: positive peak in phase A, negative peak in phase C, positive peak in phase B, negative peak in phase A, positive peak in phase C, and negative peak in phase B. These 60 degree ranges then repeat as the motor is rotated in the same direction. Each phase has a positive and negative 60 electrical degree operating range containing a peak. Each of the six ranges represents the optimum rotor position for application of current to that phase to produce torque.
Reversing voltage and current polarity into the three negative operating ranges will produce torque in the same direction as unreversed current in the three positive operating ranges. FIG. 4A illustrated a continuous rotating torque if the current polarity is switched going into the negative operating ranges. The correct sequencing of phase current into the six operating ranges to provide continuous rotating torque is called the six sequence commutation method. Current is switched to each phase in this sequence with the polarity indicated: A, -C, B, -A, C, -B (repeats).
Rotor position feedback from the three Hall effect devices indicates the rotor magnet positions relative to the winding phases. The switching amplifier uses this positional information to control when power is switched and reversed to the phase and operating range next in sequence. FIG. 4B shows the outputs from the three Hall devices labeled sensor A, B, and C. The Hall effect devices as shown indicate exactly six rotor positions and the optimum switching points. Each Hall effect device is in phase and centered with the positive and negative operating ranges of one phase.
Six MOSFET switches (FIG. 5) in the switching amplifier provide the voltage and current reversals for rotation. The Hall effect device feedback and the commutation circuitry determine the switching sequence of the six MOSFETS. FIG. 5 shows the MOSFET arrangement, and Table 1 lists the MOSFET sequence used for powering each phase. During any sequence only two pairs of switches power one phase. The remaining MOSFET pair is turned off. Six sequence commutation and motor rotation are achieved with this arrangement of MOSFETS operated according to Table 1.
The change of state of a sensor device will also be at the minimum torque point on the output torque curve for the brushless D.C. motor. When the motor is driven by applying the maximum applicable current and it, therefore, is in saturation or near saturation, that is the application of more current to the stator windings will not produce additional output torque, then the motor is in a condition where it will stall when trying to drive a given load.